1. Field of the Invention
The present invention relates to image forming apparatus in which a toner image on an image bearing member is transferred to a recording material using electrophotographic technology related to, for example, a copier and a laser printer. More particularly, the present invention relates to a technique to stably perform cleaning of a transfer material conveyance member which conveys the image bearing member and a transfer material.
2. Description of the Related Art
There has been a configuration in which an intermediate transfer belt for bearing a toner image to support various recording materials is provided and, in addition to the intermediate transfer belt, a conveying belt which conveys recording materials is provided in order to support a further variety of recording materials. In such a configuration, a nip portion is formed by the intermediate transfer belt and the conveying belt and, in the nip portion, a toner image is transferred from the intermediate transfer belt to the recording material carried by the conveying belt.
However, in the nip portion, a part of the toner is not transferred to the recording material but remains on the intermediate transfer belt. It is therefore necessary to provide a unit to clean a surface of the intermediate transfer belt. There is a possibility that the toner on the intermediate transfer belt adheres to the conveying belt between recording materials. It is therefore necessary to provide a unit to clean the conveying belt.
A related art method for cleaning the belt includes a blade member which is provided in contact with the belt.
However, if the toner is depleted at the position at which the blade and the belt are in contact with each other, there is a possibility that the frictional force becomes excessively large between the blade and the belt. Then there is a possibility that the blade is turned up. Japanese Patent Laid-Open No. 2002-072713 discloses usage of a patch image formed between paper to prevent the frictional force from becoming excessively large between a blade and a belt.
The apparatus described in Japanese Patent Laid-Open No. 2002-072713 includes a blade which is in contact with an intermediate transfer belt and a blade which is in contact with a secondary transfer roller. Two kinds of control are performed in the disclosed apparatus. One of the control is to set the polarity of the voltage to be applied to the secondary transfer portion when the patch image is made to pass the secondary transfer portion to be supplied to the secondary transfer roller to the same polarity as that of the transfer voltage. The other of the control is to set the polarity of the voltage to be applied to the secondary transfer portion when the patch image is made to pass the secondary transfer portion to be supplied to the secondary transfer roller to the polarity opposite to the polarity of the transfer voltage.
Since the toner adheres to the conveying belt basically only between paper, the toner is not easily supplied to the blade of the conveying belt. When the recording material passes the secondary transfer nip, a part of the toner is not transferred to the recording material and remains on the intermediate transfer belt (“residual toner”). That is, the toner is supplied to the blade of the intermediate transfer belt by the residual toner.
However, the amount of the residual toner varies depending on the image rate of the toner image formed on the recording material. That is, since the amount of the residual toner increases if the toner image of high image rate is formed, there is a possibility that toner supply to the blade of the intermediate transfer belt becomes excessively large. Conversely, since the amount of the residual toner decreases if the toner image of low image rate is formed, there is a possibility that toner supply to the blade of the intermediate transfer belt becomes excessively small.